Methylmalonic acidemia is an inherited metabolic disorder that affects 1:48,000 newborns and leads to neurological impairment and generalized brain tissue damage, with little or no effective treatment to date. Accumulated methylmalonate (MMA) is believed to cause partial mitochondrial electron transport chain inhibition, resulting in energetic deprivation and acidosis similar to those occurring in ischemia/reperfusion. This application is based on the hypothesis that mitochondrially-targeted drugs that protect against cell death induced by energy deprivation or ischemia/reperfusion could prove effective in preventing tissue damage in methylmalonic acidemia. The specific aims of this application are: 1. to determine the effects of MMA on mitochondrial function. 2. To evaluate the role of the permeability transition, a non-selective inner mitochondrial membrane permeabilization caused by excessive Ca2+ uptake, in mitochondrial alterations induced by MMA. 3. To assess the role of ATP-sensitive K+ channels in mitochondrial alterations induced by MMA. 4. To study the effects of mitochondrial alterations promoted by MMA and their prevention on neural cell viability and brain structure and function. These studies will involve the use of in vitro models including rat brain mitochondria, cell cultures, and rat brain slices exposed to MMA. The investigators will also use in vivo rat models involving systemic and intracerebral injections of MMA. The possible protective effects of ATP-sensitive K+ channel agonists (such as diazoxide) and permeability transition inhibitors (such as cyclosporin A) will be evaluated in all models used. Results obtained in this study will contribute toward the understanding of mechanisms of tissue damage in methylmalonic academia, and potentially determine mitochondrial targets for tissue protection in this disease.